1. Field of the Invention
This invention relates generally to data disk drive systems and more particularly to a process for determining the center of a data disk track.
2. Description of the Prior Art
In electronic computer technology, it is common to store data in binary form on the face of a rotatable disk. The face of the disk is coated with a magnetizable substance such as iron oxide. The disks are operated by rotating them like phonograph records and the binary data is encoded upon, or retrieved from, the face of the disk by a movable magnetic transducer device called a read/write head. The binary information is encoded on the face of the disk in concentric rings, called tracks, and the read/write head can move radially along the disk face to select a particular track to record or retrieve information.
These magnetic recording hard disks typically have track densities of about 1,000 tracks per inch of radius of the disk. Because of the high density, precise positioning of the read/write head is necessary so that the head can accurately gain access to a particular desired track on the surface of the disk.
A problem in exactly positioning the head over the desired track arises due to thermal expansion. The data disks are designed to operate in a temperature range of five degrees Celsius (5.degree. C.) to fifty degrees Celsius (50.degree. C.). In operation, the disk drives heat up rapidly once they have been turned on. It is conceivable that a data disk drive could be initially turned on and the disk formatted at five degrees Celsius (5.degree. C.) and later that day the disk could be operating at fifty degrees Celsius (50.degree. C.). The increased temperature causes the disk to expand and the data disk tracks are offset. The transducer head cannot be exactly positioned over the tracks to be read or written upon and errors result.
The expansion of the disk need not be great to cause problems when the data track widths are only one thousand microinches. As the track densities become greater in order to increase the storage capacity, thermal expansion becomes more of a problem.
Prior art processes have been developed to compensate for thermal expansion. Quantum makes a device called Servo-wedge which has thermal expansion compensation. The Servo-wedge data disk is encoded with factory written servo bursts between the data tracks of surface three of one of the data disks. The tracks on the data disks are divided into eight zones. When thermal compensation is needed for a particular zone, the heads are positioned in the zone, and head 3 is instructed to read the servo bursts from surface 3. If the frequency from the burst on one side shows greater than the frequency from the burst on the other side, then the controller instructs the head to move away from the stronger side toward the weaker side. The thermal offset for each zone is stored for future use.
NEC also has a thermal compensation technique. The disks are divided into five zones. Two consecutive tracks in the middle of each zone are designated as representative tracks. These tracks have servo information written in the area immediately following the index. When thermal compensation is needed, the head seeks the representative tracks in a zone. The tracks are read and the head is microstepped until the servo information read is equal to the servo information stored in the microprocessor. Another method is disclosed by U.S. Pat. No. 4,499,510 by Philip Harding and Leonard Schupak. In this method a flexible or floppy disk with ninety-six tracks per inch is used. The head is positioned near the first data track and microstepped across the track. The error correction code is checked at each microstep. When no errors are first detected, the prior microstep is labelled "L". When a first error is again detected, the microstep is labelled "H". The track center is calculated as being midway between L and H and is called "M". This operation is then repeated on all subsequent data tracks. Thus, the entire disk must be microstepped across whenever there is need to correct for thermal expansion.
Although the Harding, et al. method works for floppy disks, it does not work with hard disks. Hard disks have track densities of one thousand tracks per inch compared to the ninety-six tracks per inch for a floppy disk. The Harding, et al. method is not accurate enough to be used at these higher track densities. Also, the method of microstepping across each track, while possible for an eighty track floppy disk, is too time consuming for a one thousand track hard disk.